use stdlib's json module if simplejson is missing

json is available in Python >= 2.6
Fix typo blocking access to exmachina, and report the problem if unable to load the exmachina client library.
Change first_boot module to show and update current hostname,
instead of showing the box_name value and failing to set anything
when the user specify a host name value.
Fix expert user access checks.
Several places in the code, cfg.users.expert() is used as a boolean
test to see if the current user is an expert user.  But this do not work.
Change the implementation of expert() to assume the current user if no
argument is given, to get the code working.
Give sensible feedback for non-expert users visiting system/configure.
Explain that only expert users get access, instead of only showing
an title.
Rewrite fix for UserStore.expert() to be more like UserStoreOld.expert().
Import exmachina into plint, using source from Tom Galloway.
Fix exmachine import.
Provide a way to get out of the first-boot screen, even if it isn't ready yet.
Enable multithread for UserStore DB.
Improve message shown to the users.
Merge pull request #25 from jvalleroy/fix-login

Enable multithread for UserStore DB.
Merge pull request #22 from p1otr/master

use stdlib's json module if simplejson is missing
Merge pull request #21 from petterreinholdtsen/nonexpert-config-feedback

Give sensible feedback for non-expert users visiting system/configure.
Merge pull request #20 from petterreinholdtsen/expert-user-access

Fix expert user access checks.
Merge pull request #18 from petterreinholdtsen/find-exmachina-client-library

Fix typo blocking access to exmachina, and report the problem if unable ...
Merge pull request #19 from petterreinholdtsen/first-boot-set-hostname

Change first_boot module to show and update current hostname,
Merge pull request #24 from petterreinholdtsen/first-boot-no-dead-end

Provide a way to get out of the first-boot screen, even if it isn't read...
Merged Petter's unify-ExMachina-and-Plinth changes.
Reverted change 657068b0.